Denoising option grayed out
You should uncheck the Progressive Refine option (Under the Performance render properties panel) in order to enable and tweak Denoising settings for high quality renders.

Following are the details for this behavior.

Denoising is a post processing pass after the render tiles and layers are finished. It tries to remove fire flies and other noise by utilizing rendered data. It works best when there are enough samples rendered and saved in render buffers.

The progressive refinement display of an ongoing render is an interactive feedback tool and since it is not used for final renders but for quick turnaround renders for fine tuning wip settings, the denoising feature is disabled. 

Denoising and Render Layers in Blender